This invention relates to a method and apparatus for preparing relief printing plates using photocurable materials. More particularly, this invention relates to a method and apparatus for improving the image fidelity and character geometry of the relief image formed on such plates.
At one point in time relief printing plates were manufactured of metal, for example zinc or magnesium, by photoengraving techniques requiring relatively long etching periods for formation of the relief pattern. More recently, photocurable materials have been substituted for metals in the formulation of printing plates. In the manufacture of relief printing plates from photocurable materials, a layer of a liquid or solid material is used which undergoes Polymerization, cross-linking, or other curing reaction in response to irradiation by actinic light, usually ultraviolet light. The actinic light is passed first through a negative and then through the photocurable material to selectively cure or harden the material in a pattern corresponding to the image borne by the negative. The negative is an image bearing transparency consisting of substantially opaque and substantially transparent areas. Photocuring takes place in the exposed areas, i.e., those areas of the photocurable layer corresponding positionally to the substantially transparent areas of the negative, and no photocuring takes place in the nonexposed areas. The photocurable layer may also be exposed from the side opposite the negative. Generally, this "back exposure" is non-imagewise and used to form a hardened base for the raised printing indicia formed by the front exposure. Back exposure is typically used where the photocurable material is a liquid. The exposed layer is then developed by removal of the unexposed, unhardened portions with an air knife, developer solvent, or other means to form a relief image.
In producing relief images from half-tone negatives, it is required to produce in the relief relatively small diameter raised printing indicia in what are otherwise recessed areas of the relief. These areas print the lighter or "highlight" areas of the image and the raised indicia in these areas and are accordingly referred to as "highlight dots". The size and density of highlight dots control shading or tone in the light image areas. The half-tone relief image also generally contains relatively shallow, small diameter depressions in what are otherwise overall raised areas of the relief. These areas print the darker or "shadow" areas of the image and the depressions are generally referred to as "shadow reverses". As with the highlight dots, the size and density of shadow reverses control the tone in the darker image areas.
The production of satisfactory highlight dots and shadow reverses in relief images by photoexposure techniques presents unique problems. Due to the small size of the transparent dots in the half-tone negative used to produce highlight dots, it is generally desired to provide a high image exposure dose in order to assure formation of the dots and adequate depthwise curing. Such curing is needed to anchor the dot to the image base. However, a high radiation dose has an adverse effect on the formation of shadow reverses. Shadow reverses are formed beneath relatively small opaque areas of the negative in an otherwise predominantly transparent area. A high exposure dose can result in overexposure of these areas, i.e., beneath the opaque areas, thus resulting in unwanted curing of the photocurable material and filling in of the shadow reverses.
The high image exposure dose also results in overexposure in the peripheral areas outside of the transparent dot areas of the half-tone negative. This results in increased dot image area, i.e., the relief dot is larger than the corresponding transparent areas of the negative. In addition, the overexposure results in a higher, broader shoulder profile on the dot. This leads to the printing of smudges and larger than intended dots, particularly where there is any overimpression during printing.
Although a lower radiation dose can be used to lessen the filling in of shadow reverses and improve dot shoulder geometry, the lower dose often results in unsatisfactory formation and anchoring of the highlight dots. In addition, it has been observed that the highlight dots have a substantially greater tendency to move or otherwise deform during the photoexposure step when lower intensity radiation is used. Such dot movement or deformation is not fully understood but is believed to be due, at least in part, to shrinkage of the photocurable material as curing occurs. Dot movement is highly undesirable, resulting not only in positional displacement of the dot but also the formation of a streak or tail behind the dot, and in the direction of movement, which becomes part of the relief image and adversely affects the quality of images printed with the plate.
For many photocurable systems, there is no exposure dose which satisfactorily provides both highlight dots and shadow reverses, and the operator is thus forced to make undesired compromises in exposure dose and image quality. For other systems, there may be an exposure dose which provides generally acceptable highlight dots and shadow reverses, but this almost always occurs in a very narrow "window" of doses, thus attaching a very high degree of criticality to the exposure. The narrow window also means a high risk of error in the exposure step, particularly where there is variability in the intensity of the actinic radiation or in the photoresponse of the photocurable material from one lot to another.
Although the foregoing focuses on the problems with image fidelity and resolution encountered in preparing half-tone relief images, it will be appreciated that similar problems can occur in the formation of other types of high resolution relief images, such as line images. It should also be appreciated that these problems are unique to the formation of relief images by photoexposure, deriving from the fact that the photoexposure must provide an image having not only length and breadth dimensions, but also a substantial and significant depth dimension. The necessity of forming this third image dimension places unique demands on the photosensitive system and the photoexposure method which are not encountered in those processes and systems used to form only two dimensional images, such as in conventional photographic or photocopying systems.